Narrow spectral width high power distributed feedback semiconductor lasers

ABSTRACT

High power edge emitting semiconductor lasers are formed to emit with very narrow spectral width at precisely selected wavelengths. An epitaxial structure is grown on a semiconductor substrate, e.g., GaAs, and includes an active region at which light emission occurs, upper and lower confinement layers and upper and lower cladding layers. A distributed feedback grating is formed in an aluminum free section of the upper confinement layer to act upon the light generated in the active region to produce lasing action and emission of light from an edge face of the semiconductor laser. Such devices are well suited to being formed to provide a wide stripe, e.g., in the range of 50 to 100 μm or more, and high power, in the 1 watt range, at wavelengths including visible wavelengths.

This application is a divisional of application Ser. No. 09/067,189,filed Apr. 27, 1998, now U.S. Pat. No. 6,195,381.

This invention was made with United States government support awarded bythe following agencies: DOD-AF, Grant No.: F49620-96-C-0052; and NSFGrant No.: 9522035 NSF Grant No.: 9612244; NSF Grant.: 9522667; and NSFGrant No.: 9531011. The United States has certain rights in thisinvention.

FIELD OF THE INVENTION

This invention pertains generally to the field of semiconductor diodelasers and particularly to edge emitting distributed feedbacksemiconductor lasers.

BACKGROUND OF THE INVENTION

Semiconductor diode lasers are formed of multiple layers ofsemiconductor materials. The typical semiconductor diode laser includesan n-type layer, a p-type layer and an undoped active layer between themsuch that when the diode is forward biased electrons and holes recombinein the active region layer with the resulting emission of light. Thelayers adjacent to the active layer typically have a lower index ofrefraction than the active layer and form cladding layers that confinethe emitted light to the active layer and sometimes to adjacent layers.Semiconductor lasers may be constructed to be either edge emitting orsurface emitting. In an edge emitting Fabry-Perot type semiconductorlaser, crystal facet mirrors are located at opposite edges of themulti-layer structure to provide reflection of the emitted light backand forth in a longitudinal direction, generally in the plane of thelayers, to provide lasing action and emission of laser light from one ofthe facets. Another type of device, which may be designed to be eitheredge emitting or surface emitting, utilizes distributed feedbackstructures rather then conventional facets or mirrors, providingfeedback for lasing as a result of backward Bragg scattering fromperiodic variations of the refractive index or the gain or both of thesemiconductor laser structure.

Semiconductor lasers having CW power in the watt-range and narrowbandwidth, e.g., less than 2 Å full width half maximum (FWHM), would bedesirable for a variety of applications. Examples include 0.894 μm diodelasers which may be used for polarizing Cs to generate spin-polarized Xegas for magnetic resonance imaging, low-chirp pump sources for solidstate lasers, and in spectroscopy sources for monitoring environmentalgases. Conventional broad stripe (≧25 μm) semiconductor lasers used forobtaining high powers typically have a spectral width of about 20 Å FWHMor more at high drive levels and broaden further under quasi-CWoperation. Significant improvements in spectral width can be obtainedusing distributed feedback (DFB) gratings or distributed Braggreflectors (DBR) rather than Fabry-Perot mirror facets for opticalfeedback. 278 mW CW power with about 1 Å of wavelength variation,resulting from mode hopping, has been reported for narrow-stripe DBRlasers. J. S. Major, et al., Electron. Lett. Vol. 29, No. 24, p. 2121,1993. Using DFB phase-locked laser arrays, narrow bandwidth operationhas been obtained from large apertures at relatively long wavelengths(λ=1.3 μm to 1.5 μm). 120 mW pulsed operation has been reported from a45 μm aperture device (λ=1.3 μm), Y. Twu, et al., Electron. Lett. Vol.24, No. 12, p. 1144, 1988, and 85 mW CW from a 72 μm aperture device(λ=1.55 μm), K. Y. Liou, et al., Tech. Dig. 13th IEEE Int. Semicond.Laser Conf., Paper D7, 1992. For applications where (lateral) spatialcoherence is not necessary, a broad-stripe laser with a DFB grating isapparently well suited for achieving high CW powers with narrow spectrallinewidth.

A limitation is encountered with DFB lasers designed to operate atshorter wavelengths including visible light wavelengths, in thatconventional diode lasers grown on GaAs substrates, which can emit inthe range of wavelengths between about 0.6 μm to 1.1 μm, generally haveoptical confinement layers containing aluminum as well as claddinglayers containing aluminum. Due to the high reactivity of aluminum(i.e., essentially instant oxidation when exposed to air), it has provento be very difficult to make single frequency lasers of the DFB type inthe foregoing wavelength range in which the grating is buried within themulti-layer semiconductor structure. Consequently, the commerciallyavailable high power, narrow linewidth lasers have been of thedistributed Bragg reflector (DBR) type, in which the grating is outsideof the active lasing part of the structure. However, such DBR devicessuffer from the major drawback of mode hopping that occurs withincreasing drive current due to changes in the lasing-region index ofrefraction with increasing drive power.

SUMMARY OF THE INVENTION

The present invention encompasses a high power edge emittingsemiconductor laser with very narrow spectral width that can be tailoredto operate at precisely selected wavelengths including wavelengths inthe visible range. In accordance with the invention, typical CW powersin the watt range are obtainable with a narrow linewidth of 2 Å FWHM orless. Consequently, such lasers are well suited to applicationsrequiring precise narrow linewidth laser sources, such as for polarizingcesium or rubidium for use in magnetic resonance imaging with spinpolarized xenon.

The edge emitting semiconductor laser of the invention includes asubstrate and an epitaxial structure preferably grown on orientation onthe substrate. The epitaxial structure includes a layer with an activeregion at which light emission occurs, upper and lower confinementlayers adjacent the active region layer, upper and lower cladding layersadjacent the confinement layers, outer edge faces perpendicular to theactive region layer, and electrodes by which voltage can be appliedacross the epitaxial structure and the substrate. A distributed feedbackgrating is formed on an aluminum free section of the upper confinementlayer. The grating is comprised of periodically alternating elementsdiffering from one another in dielectric constant, and thus generally inindex of refraction, to provide optical feedback for a selectedeffective wavelength of light generation from the active region. Becausethe distributed feedback grating in accordance with the invention isformed in a layer above the active region, regrowth problems and thepropagation of dislocations that are encountered with gratings formedbelow the active region layer are avoided. In addition, it has beenfound, in accordance with the invention, that by utilizing a confinementlayer at least a section of which is aluminum free, the grating may bereadily etched in the aluminum free confinement layer to provide agrating surface on which additional epitaxial layers may be grownwithout difficulty. Such devices are well-suited to being formed toprovide a wide emitting aperture, preferably at least 25 μm to providehigh power lasing, which may be defined by current confinement.

The invention may be incorporated in semiconductor lasers having a GaAssubstrate and epitaxial layers (preferably grown on (100) orientation onthe substrate) including an active region layer with single or multiplequantum wells of InGaAs surrounded by InGaAsP barrier layers, opticalconfinement layers of InGaP, with the distributed feedback gratingformed in the top surface of the upper InGaP confinement layer, andcladding layers of InGaAlP or AlGaAs. The thickness of the upperconfinement layer and the spacing of the grating from the active regionlayer is preferably at least about 0.2 μm to ensure small coupling tothe grating. Small grating coupling coefficient, κ, is needed tomaintain a κL product of about unity, where L is the cavity lengthbetween the edge faces of the laser, which, in turn, ensures bothefficient DFB laser operation as well as single-longitudinal-modeoperation to high drive levels above threshold. Since watt-range lasersrequire long cavities (L≧1 mm), to keep κL˜1 it is of criticalimportance to have a low κ value. Such structures can be formed tooperate in the range of 1 watt CW with a linewidth of less than 1 Å andat 1 watt pulsed (5 μs-wide pulses) with a linewidth of 1.2 Å. Becausethe upper confinement layer of InGaP is aluminum free, it may be etchedin a conventional manner to leave a surface of the grating on whichregrowth is readily accomplished.

Further objects, features and advantages of the invention will beapparent from the following detailed description when taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic diagram illustrating an example of a compositionfor the active region layer and adjacent confinement and cladding layersin accordance with the invention.

FIG. 2 is a perspective view of an exemplary semiconductor laser formedin accordance with the invention.

FIG. 3 are plots of CW power and wallplug efficiency as a function ofdrive current for the exemplary device of FIG. 2.

FIG. 4 is a plot of CW emission wavelengths for the exemplary device ofFIG. 2.

FIG. 5 are plots of emission wavelengths for pulsed operation (quasi-cw)of the exemplary device of FIG. 2.

FIG. 6 is a simplified view of the detailed multi-layer structure of anexemplary device in accordance with the invention.

FIG. 7 is an illustrative diagram of an alternative exemplarycomposition for a semiconductor laser in accordance with the invention.

FIG. 8 is an illustrative diagram of an alternative composition for asemiconductor laser which has an asymmetric transverse opticalwaveguide.

DETAILED DESCRIPTION OF THE INVENTION

For purposes of illustrating the present invention, a diagramillustrating an exemplary multi-layer waveguide structure in accordancewith the invention is shown in FIG. 1 along with a diagram of the bandgap energy for the several layers. The exemplary structure of FIG. 1includes an active region layer 10 including double quantum wells 11 ofInGaAs surrounded by InGaAsP barrier layers 12, a lower confinementlayer 14 and an upper confinement layer 15, both formed of InGaP, and alower cladding layer 17 of n-type InGaAlP and an upper cladding layer 18of p-type InGaAlP adjacent to the lower and upper confinement layers,respectively. A distributed feedback grating 20 is formed at the top ofthe upper confinement layer 15. The upper confinement layer is formed ofInGaP and is free of aluminum. Thus, once the grating 20 has been formedin the upper confinement layer 15, the upper cladding layer 18 andsubsequent layers may be readily grown over the grating. The right-handside of FIG. 1 is a diagram illustrating the band gap energy levels ofthese various layers.

A simplified perspective view of a semiconductor laser incorporating thewaveguide layers of FIG. 1 is shown in FIG. 2. The layers illustrated inFIG. 2 are epitaxially grown on a GaAs substrate 22. It is preferredthat the top surface 23 of the substrate 22 be the (100) surface andthat the epitaxial layers be grown on this surface exactly onorientation. For exemplification only, current confinement may beprovided to define the emitting aperture by insulating layers 26 of SiO₂over a cap layer 25 of p+ GaAs, with a top metal electrode 27 in contactwith the cap layer 25 at the top face of the laser between theinsulating SiO₂ layers to provide electrical conduction thereto. Abottom metal electrode 29 is formed on the bottom face of the substrate22 so that voltage may be applied across the semiconductor laser betweenthe electrodes 27 and 29. The width W of the metal electrode 27 incontact with the cap layer 25 defines the effective stripe width of thesemiconductor diode in the active layer 10 at which light emission willoccur.

A specific example of such a structure is a 100 μm wide stripe laserwhich operates at 1.1 W CW with a linewidth of 0.9 Å, and at 1 Wquasi-CW (5 μs pulse width at 2 kHz repetition rate) with a linewidth of1.2 Å. The double quantum well layers 11 are formed ofIn_(0.03)Ga_(0.97)As between and separated by InGaAsP (bandgapenergy=1.62 eV) barrier layers 12, with the optical confinement layers14 and 15 formed of In_(0.47)Ga_(0.53)P, and with the cladding layers 17and 18 formed of In_(0.5) (Ga_(0.5)Al_(0.5))_(0.5)P. The large bandgapof the In_(0.5) (Ga_(0.5)Al_(0.5))_(0.5) P cladding layers 17 and 18ensures good carrier confinement for these structures, resulting inhigher internal efficiencies than would be obtained from an entirelyaluminum free structure. The grating 20 is a second-order grating thatis holographically patterned and wet etched directly into the InGaPupper confinement layer 15 over the active region. As an example, thegrating, which may have a sinusoidal form, can have a period betweenadjacent peaks of the periodic elements of about 2740 Å and apeak-to-peak depth of about 500 Å. Because InGaP is less prone tooxidation than aluminum containing compounds, regrowth over the gratingis readily accomplished. Thus, the InGaAlP upper cladding layer 18 issimply grown over the grating 20, followed by the p+ GaAs cap layer 25.This structure can be designed, for example, to lase at 0.894 μm.Multiple oxide stripe broad area laser structures may be processed fromthis material by cleaving the bars perpendicularly to the stripe alongthe crystal facets to provide an emitting edge face 31 and an oppositereflecting edge face 32 to produce 1 mm-long lasers with, e.g., 5%reflectivity at the emitting edge face 31 and 95% facet reflectivity atthe reflecting edge face 32.

The CW power-current curve 35 for a 1 mm-long, 100 μm-wide laser inaccordance with the invention at 10° C. as shown in FIG. 3, andillustrates that the threshold current density, J_(th), is 240 A/cm²,the differential quantum efficiency, η_(d), is 51%, and the wallplugefficiency curve 36 shown in FIG. 3 illustrates that the wallplugefficiency ηp reaches a maximum value of 32% at 1.1 W (7.3 timesthreshold). By comparison, devices made without the distributed feedbackgrating 20 but with otherwise the same structure and dimensions have, at20° C., a J_(th) of 225 A/cm² and η_(d) of 62% with characteristictemperatures T₀=200K and T₁=480K.

As illustrated in FIG. 4, the spectrum of the broad-area DFB laser ofthe invention appears to be single frequency near threshold with atemperature dependence of 0.6 Å/C and maintains a narrow linewidth at 1W output power as shown by the right-hand peak in FIG. 4. The FWHM forthe CW spectrum at 0.53 W is 0.5 Å FWHM. At 1.1 W, if a width isapproximated based on the envelope of the peaks, the spectrum broadensto about 0.9 Å FWHM. Under quasi-CW conditions, as illustrated in FIG.5, the spectrum is broader than the CW spectrum, which can be attributedto thermal-induced and carrier-induced transients (chirp). The spectrameasured for 5 μs pulses at a frequency of 2 kHz yields widths of 0.9 Åand 1.2 Å FWHM at 0.5 W and 1.0 W, respectively, as shown by the lowerand upper spectra diagrams in FIG. 5. In contrast, the spectra nearthreshold of Fabry-Perot lasers have a width of over 10 Å FWHM and reach20 Å FWHM at 1 W CW. The angular FWHM of a lateral farfields for thebroad area DFB lasers of the invention is approximately 4° nearthreshold, 5° at 0.5 W, and 6.5° at 1.0 W under both CW and pulseconditions, indicating that some spatial mode discrimination occurs ascompared to the Fabry-Perot devices which have 8° FWHM farfields at lowdrive levels.

An exemplary detailed fabrication process for the large aperture DFBlasers of the invention is given below, and forms the epitaxial layerstructure on a GaAs substrate as illustrated in FIG. 6.

The grating base structure is grown in a low-pressure (50 mbar) metalorganic chemical vapor deposition (MOCVD) reactor at 700° C. Thesubstrate 22 is an epiready n+ GaAs substrate on orientation, (100). Asillustrated in FIG. 6, the following epitaxial layers (with exemplarythicknesses shown) are grown in sequence on the substrate (includingsuitable buffer and transitional layers in addition to the mainfunctional layers shown in FIG. 2): n-GaAs buffer layer 40; n-InGaPtransition layer 41 (lattice matched);n-In_(0.5)(Ga_(0.5)Al_(0.5))_(0.5)P lower cladding layer 17 (latticematched) InGaP optical confinement layer 14 (lattice matched); InGaPslow growth rate layer 43 (lattice matched; InGaAsP transition barrierlayer 12 (lattice matched—Eg=1.62 eV); InGaAs quantum well 11; InGaAsPbarrier layer 12 (lattice matched—Eg=1.62 eV); InGaAs quantum well 11;InGaAsP transition barrier layer 12 (lattice matched—Eg=1.62 eV); InGaPslow growth rate layer 44 (lattice matched); InGaP optical confinementlayer 15 (lattice matched). All n-type layers are Si doped.

The gratings are holographically defined in photoresist and thentransferred into the exposed InGaP confinement layer 15 using wetchemical etching. The surface of the layer 15 is cleaned in a HF:DIwater (1:10) solution for 30 seconds. It is then rinsed in a DI(deionized) water solution for 5 minutes and blown dry with nitrogengas. A solution of Shipley 1805 photoresist:Shipley Type P thinner (1:2)is spin coated onto the sample at 5000 rpm for 30 seconds. The coatedsample is then baked in an oven at 90° C. for 30 minutes.

The photoresist is exposed by light from an Ar-Ion laser. The lighttravels through a 50%/50% beam splitter. Each leg of the split beam goesthrough a spatial filter to generate diverging coherent spherical waves.The light from each of the two beams is then reflected onto the sample.The sample is aligned such that the periodic lines produced by the laserare parallel to the crystal plane that forms the cleaved facet of thelaser. The laser is set to 200 mW, with the power density of each legmeasured at roughly 30 mW/sq. cm at the sample. The sample is exposedunder these condition for a time of 60 sec.

The exposed photoresist is then spray developed using Shipley ME-321developer on a spinner rotating at 2000 rpm for a time of 10 sec. Thesample is then spray rinsed with DI water at 2000 rpm for 1 minute, andallowed to spin dry at 2000 rpm for 1 minute. The InGaP is etched in asolution of [Br₂:HBr (1:17)]:DI (1:80) for 20 seconds and rinsed in DIwater for 10 minutes. The sample is then blown dry with nitrogen gas.The sample is stripped in Shipley 1165 photoresist stripper for 5 minand rinsed in DI water for 10 minutes. The surface is then cleaned inacetone for 1 minute and methanol for 1 minute. This is followed byrinsing in DI water for 3 minutes and blowing the sample dry withnitrogen gas. The surface is treated with a mild oxygen plasma etch in aPlasmatherm etcher. 25 sccm of O₂ gas flows into the reactor maintainedat a pressure of 20 mT. A 100W plasma is excited for 4 minutes. Thesample is etched in a buffered oxide etch (BOE)—NF₃:HF (20:1) for 30seconds, and rinsed in DI water for 5 minutes. The sample is then blowndry with nitrogen gas.

The upper cladding layer and a highly doped cap are then grown over thegrating using the same MOCVD system. The In compounds are grown at 700°C. and are Zn-doped. The first GaAs layer is grown at 625° C. and isZn-doped. The last layer is grown at 575C and is C-doped. The followinglayers are grown in sequence: p-In_(0.5)(Al_(0.5)Ga_(0.5))_(0.5)P uppercladding layer 18 (lattice matched); p-InGaP transition layer 46(lattice matched); p-GaAs layer 47; p+-GaAs highly doped cap 25 (p˜10²⁰cm⁻³).

The following process is used to produce a broad-stripe current confinedlaser using oxide isolation. Of course, current confinement may beprovided in a conventional manner via back-biased p-n junctions, protonimplantation, etc., and lateral index guiding may also be utilized ifdesired. Shipley 1805 photoresist is spin coated onto the sample at 5000rpm for 30 seconds. The coated sample is baked in an oven at 90° C. for30 min. The photoresist is exposed in a Karl-Suss mask aligner with afirst mask. The sample is aligned such that the stripes of the mask areperpendicular to the crystal plane that forms the cleaved facet of thelaser. The exposed photoresist is developed in Shipley MF-321 with mildagitation for 1 minute. The sample is then rinsed with DI water for 3minutes, and blown dry with nitrogen gas. The developed sample is bakedin an oven at 110° C. for 30 min. The GaAs cap 25 is etched in aNH₄OH:H₂O₂:DI water (3:1:50) solution for 1 minute. It is then rinsed ina DI water solution for 5 minutes and blown dry with nitrogen gas. Thesample is then stripped in Shipley 1165 photoresist stripper for 5 min.and rinsed in DI water for 10 minutes. The surface is cleaned in acetonefor 1 minute and methanol for 1 minute. This is followed by rinsing inDI water for 3 minutes and blowing the sample dry with nitrogen gas. Thesurface is then coated with an 80 nm SiO₂ film deposited in aPlasmatherm plasma enhanced chemical vapor deposition (PECVD) reactor.The chamber is heated to 60° C. and the substrate to 250° C. N₂O andSiH₄ (2% in N₂) are flowed at rates of 810 sccm and 440 sccm,respectively, into the reactor maintained at a pressure of 900 mT. A 30Wplasma is excited for 100 seconds.

Shipley 1805 photoresist is then spin coated onto the sample at 5000 rpmfor 30 seconds. The coated sample is baked in an oven at 90° C. for 30minutes. The photoresist is exposed in a Karl-Suss mask aligner with asecond mask. The sample is aligned such that the contact stripe iscentered between the isolation grooves. The exposed photoresist isdeveloped in Shipley MF-321 with mild agitation for 1 minute. The sampleis then rinsed with DI water for 3 minutes, and blown dry with nitrogengas. The developed sample is baked in an oven at 110C. for 30 min.

The SiO₂ layer 26 is etched in a buffered oxide etch (BOE)—NF₃:HF (20:1)for 1 minute, and rinsed in DI water for 5 minutes. The sample is thenblown dry with nitrogen gas. The sample is stripped in Shipley 1165photoresist stripper for 5 min and rinsed in DI water for 10 minutes.The surface is cleaned in acetone for 1 minute and methanol for 1minute. This is followed by rinsing in DI water for 3 minutes andblowing the sample dry with nitrogen gas. The p-side metal contactelectrode 27 is deposited using an electron beam evaporator. A threemetal contact is used consisting of Ti(20 nm), Pt(50 nm), and Au(300nm). The sample is wax mounted p-side down to a glass plate and thinnedto 100 μm by mechanical lapping with 9 μm slurry. The sample is thenreleased and rinsed with acetone to remove the remaining wax. Thesurface is cleaned in acetone for 1 minute and methanol for 1 minute.This is followed by rinsing in DI water for 3 minutes and blowing thesample dry with nitrogen gas. The n-side metal contact 29 is depositedon the back side of the sample using an electron beam evaporator. A fourmetal contact is used consisting of Ge(10 nm), AuGe alloy(100 nm), Ni(30nm), and Au(200 nm). The sample is heated in forming gas (10.5% H₂ inN₂) in a rapid thermal annealer (RTA). The heat cycle is 375C for 30seconds. The sample is then scribed at the edge and cleaved in bars. Thecleaved edge faces 31 and 32 are perpendicular to the laser stripes, andform the reflecting facets of the laser cavity. The front and back edgefacets 31 and 32, respectively, of the lasers are coated with ananti-reflective (AR) and highly-reflective (HR) dielectric layers,respectively. A typical AR coating consists of a quarter-wave thicklayer of Al₂O₃. The HR coating may consist of, for example, multiplepairs of quarter-wave SiO₂ and Si layers. These layers can be depositedin an electron beam evaporator. The bars may then be cut into individualdevices by scribing lines between the stripes and breaking the bar intodevices, or dicing the chips with a diamond saw.

It is understood that the particular multi-layer structure describedabove is not the only structure in which the present invention may beembodied and that the invention is not limited to that structure. Anexample of a modified embodiment is illustrated in FIG. 7 in which afirst section 50 of an optical confinement layer 15 of InGaP is formedabove the active region layer 10, followed by an intermediate opticalconfinement layer section 51 of InGaAlP. A further section of the upperconfinement layer 15, formed of aluminum free InGaP 53, is then formedover the layer 51, and the grating 20 is then formed as discussed aboveon the surface of the aluminum free layer 53. Many other variations onthis structure are possible. For example, the upper and lowerconfinement layers adjacent to the active region layer may be formed ofInGaAlP, with the aluminum free section of the upper confinement layerthen being formed over the upper layer of InGaAlP. Further, the uppercladding layer can be AlGaAs instead of InGaAlP. It is also understoodthat other optical confinement layer materials may be utilized, such asInGaAsP for wavelengths greater than about 0.8 μm and GaAs forwavelengths greater than about 0.92 μm. For structures in which thealuminum free InGaP section of the upper confinement layer is formedover a layer of InGaAlP, it is preferred that the layer of InGaAlP isrelatively thick, e.g., greater than about 0.2 μm, so that a small partof the optical mode will “see” the grating. This has the advantage ofsmall coupling to the grating, allowing for high output powers, and anydamage at the grating interface does not affect the device performance.

Another example of a modified structure is illustrated in FIG. 8 inwhich the optical confinement layers 14 and 15 and the cladding layers17 and 18 are formed to provide an asymmetric transverse opticalwaveguide supporting only the fundamental transverse mode. The lowercladding layer 17 has an index of refraction higher than that of theupper cladding layer 18, which causes the optical mode to have both lowoverlap with the grating layer 20 as well as low overlap with the activeregion 10. The field intensity profile is illustrated by the linelabeled 60 in FIG. 8. Thus, this structure simultaneously provides thedesired small coupling to the grating and a small transverse opticalconfinement factor, Γ, which ensures a large equivalent transverse spotsize for high power operation.

Other material systems may be used for the quantum wells of the activeregion layer. One further example of a material system for the quantumwells is In_(1−x)Ga_(x)As_(y)P_(1−y), where 0<x<1 and 0<y<1.

It is understood that the invention is not confined to the particularembodiments set forth herein as illustrative, but embraces all suchmodified forms thereof as come within the scope of the following claims.

What is claimed is:
 1. A method of forming a semiconductor lasercomprising: (a) providing a semiconductor substrate having a surface onwhich epitaxial layers may be formed; (b) growing on the surface of thesubstrate at least a lower cladding layer, then a lower confinementlayer, their an active region layer in which light emission occurs, andthen an upper confinement layer that is free of aluminum; (c) thenetching a distributed feedback grating structure on the aluminum freeupper confinement layer comprising periodically alternating elements;and (d) then growing an upper cladding layer over the grating etched inthe upper confinement layer to provide a grating in which the adjacentelements in the grating differ from one another in dielectric constantso as to provide optical feedback for a selected wavelength of lightgeneration from the active region.
 2. The method of claim 1 wherein thesubstrate is GaAs and the eptixial structure is grown on orientation ona (100) surface of the GaAs substrate.
 3. The method of claim 2 whereinthe upper and lower cladding layers are InGaAlP or AlGaAs, the upper andlower confinement layers are InGaP, and wherein the step of growing theactive region layer includes growing barrier layers of InGaAsP andgrowing at least one quantum well layer of InGaAs between the barrierlayers.
 4. The method of claim 1 wherein the epitaxial structure isgrown on the substrate by metal organic chemical vapor deposition.